Field of the Invention
The present invention relates to a radiation detection apparatus and a radiation detection sheet to be used for a medical image diagnostic apparatus, a nondestructive inspection apparatus, an analysis apparatus, or the like.
Description of the Related Art
With recent progress in CMOS technology using crystalline silicon and TFT technology using amorphous silicon or an oxide, various radiation detectors particularly including a two-dimensional flat panel sensor have been proposed, and a large-area high-speed digital detector has been under development also in a medical imaging field and a nondestructive inspection field.
The flat panel sensor allows an image to be instantaneously displayed on a display after or during irradiation of radiations. The flat panel sensor also allows the image to be extracted as digital information, and therefore has such a feature as to be convenient for archiving, processing, and transmitting data. Thus, the flat panel sensor has come to be widely used.
Particularly in the flat panel sensor used in the medical imaging field, a modulation transfer function (MTF) value characteristic, which reflects a resolving power, and a detective quantum efficiency (DQE) value, which indicates an S/N ratio, are key indices. Both values indicate that the flat panel sensor has more satisfactory characteristics as the values are larger, that is, are closer to 1.
The generally used flat panel sensor is of such a type as to convert radiation into light and then read the light with a light sensor to obtain an image. A layer configured to convert radiation into light like this is called “scintillator layer”, and a GOS (Gd2O2S:Tb) sheet or a CsI (CsI:Tl) needle-shaped crystal film is generally used as the layer. The GOS sheet is obtained by processing a phosphor powder of Gd2O2S:Tb together with an organic binder to have a sheet shape, and an Al reflection film is normally formed on a side opposite to a sensor side to improve a light emission luminance. As the CsI needle-shaped crystal film, a needle-shaped crystal film is used. The needle-shaped crystal film is obtained by co-depositing CsI and TlI, which is an activator to be an emission center, to grow a large number of CsI:Tl needle-shaped crystals. The needle-shaped crystal film is capable of efficiently propagating light in a needle-shaped crystal direction, which may lower a probability that the emitted light is blurred in a horizontal direction. In regard to the CsI needle-shaped crystal film, a method of directly growing the CsI needle-shaped crystal film on a light sensor and then forming an Al reflection film thereon or a method of, in contrast, growing the CsI needle-shaped crystal film on a substrate with a reflection film and then bonding the substrate to a light sensor base is employed.
In FIG. 2, a general radiation detection apparatus according to a related art is illustrated. Although not shown, in this case, a radiation source is present in an upper part of FIG. 2, that is, above a reflection layer 15 in FIG. 2. In FIG. 2, the reflection layer 15 is formed on a radiation source side of a scintillator layer 21 so as to be adjacent thereto, while a light sensor layer 16 obtained by arranging a plurality of light sensors 18 in a substrate 17 is formed on a side opposite to the radiation source. In FIG. 2, a protective layer for the light sensor or an adhesion layer between the respective layers is not shown.
In order to improve the DQE value, a method of raising the rate of stopping of radiation, namely, the absorptivity thereof by increasing the film thickness of the scintillator layer 21 is generally employed. However, as the scintillator layer 21 becomes thicker, the degree of diffusion of emitted light which is spread before reaching the sensor becomes higher, which leads to a problem of reduction in the MTF value.
On the other hand, in order to improve the MTF value, there is a method of reducing influence of scattered light by reducing the thickness of the scintillator layer 21. However, with this method, the stopping power for radiation is lowered, which results in reduction in the DQE value.
It should be understood that it is preferred that the radiation detection apparatus have a satisfactory MTF, namely, a large MTF, and a general method of improving the MTF is to reduce the thickness of the scintillator layer as described above. Such reduction allows the light sensor to detect the light before the light diffusion in the scintillator layer becomes considerable. However, with this method, the scintillator layer becomes thinner, which raises a problem in that X-rays are not sufficiently absorbed. When the absorption of the X-rays is insufficient, the DQE is lowered.
Further, as for another method of improving the MTF, in Japanese Patent Application Laid-Open No. 2008-51793, there is described a radiation detection apparatus having such a feature that the concentration of the activator is high on a radiation incident side and low on a light sensor side. Further, on the contrary, in Japanese Patent Application Laid-Open No. 2012-159393, there is disclosed a radiation image detection apparatus configured such that a region having a high activator concentration, in which the activator concentration is higher than the activator concentration in a region within a scintillator on a side opposite to a radiation incident side thereof, is formed in a position within the scintillator on the light sensor side thereof.
However, no technology relating to a method of improving the DQE is described in Japanese Patent Application Laid-Open No. 2008-51793 or disclosed in Japanese Patent Application Laid-Open No. 2012-159393.